As indicated in U.S. Pat. No. 4,078,286, in a typical process for electrophotographic duplication, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member, and the latent image is subsequently rendered visible by the application of electroscopic particles, which are commonly referred to as toner. The visible toner image is then in a loose, powdered form and it can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or another support such as a sheet of plain paper. A principle aspect of the present invention relates to the fusing of the toner image upon a support.
The use of thermal energy for fixing toner images onto a support member is well known. In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a flat or curved plate member in pressure contact with a roll; a belt member in pressure contact with a roll; and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to affect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont de Nemours under the trademark TEFLON. The silicone rubbers which can be used as the surface of the fuser member can be classified into three groups according to the vulcanization method and temperature, i.e. room temperature vulcanization silicone rubber hereinafter referred to as RTV silicone rubber, liquid injection moldable or extrudable silicone rubber, and high temperature vulcanization type silicone rubber, referred to as HTV rubber. All these silicone rubbers or elastomers are well known in the art and are commercially available.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate or belt members. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the material being copied there. The so called "hot offset" occurs when the temperature of the toner is raised to a point where the toner particles liquify and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface which has a low surface energy to provide the necessary release. While many materials may initially function with good release properties with continued use, they tend to be contaminated with paper fibers, debris and toner as a result of hot offset of toner, thereby increasing the surface energy of the roll and perpetuating the destruction of release performance. In addition, once the roll becomes contaminated the hot offset temperature starts to reduce and may reach a level near or below the minimum temperature necessary to fuse the toner image thereby leading to both incomplete fusing of the toner image and offsetting of the toner image to the fuser roll. Once the fuser roll begins to be contaminated, contaminates are likely to be transferred to the pressure roll because it is generally of a higher surface energy material.
These problems have lead to the use of the application of release agents to the fuser members to insure that the toner is completely released from the fuser roll during the fusing operation. However, the application of release agents requires a means to continuously supply a thin film of the release oil which is typically a silicone oil, particularly polydimethyl silicone oil, thereby requiring additional, expensive components, periodic maintenance of the release agent management system, as well as periodic replenishment of the release agent fluid. In addition, such systems may typically comprise a fuser roll made of a silicone elastomer as well as a release agent made from a silicone oil which frequently leads to the swelling of the elastomer by the oil and the subsequent failure of the silicone elastomer in the fusing operation.
Accordingly, there is a continuing need for dry release hot roll fuser members.